function foo() {
    console.log(this.a);
}

var a = 1

const obj = {
    a: '2',
    foo: foo
}

obj.foo()

const c = new foo()


function a() {
    return () => {
        return () => {
            console.log(this);
        }
    }
}

// console.log(a()()());

let a = {}
let fn = function() {console.log(this);}
fn.bind().bind(a)
// fn绑定在了window，第一次绑定